'\"
'\" Copyright (c) 1990-1994 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\" 
.TH Tk_CreateSelHandler 3 4.0 Tk "Tk Library Procedures"
.so man.macros
.BS
.SH NAME
Tk_CreateSelHandler, Tk_DeleteSelHandler \- arrange to handle requests for a selection
.SH SYNOPSIS
.nf
\fB#include <tk.h>\fR
.sp
\fBTk_CreateSelHandler\fR(\fItkwin, selection, target, proc, clientData, format\fR)
.sp
\fBTk_DeleteSelHandler\fR(\fItkwin, selection, target\fR)
.SH ARGUMENTS
.AS Tk_SelectionProc clientData
.AP Tk_Window tkwin in
Window for which \fIproc\fR will provide selection information.
.AP Atom selection in
The name of the selection for which \fIproc\fR will provide
selection information.
.AP Atom target in
Form in which \fIproc\fR can provide the selection (e.g. STRING
or FILE_NAME).  Corresponds to \fItype\fR arguments in \fBselection\fR
commands.
.AP Tk_SelectionProc *proc in
Procedure to invoke whenever the selection is owned by \fItkwin\fR
and the selection contents are requested in the format given by
\fItarget\fR.
.AP ClientData clientData in
Arbitrary one-word value to pass to \fIproc\fR.
.AP Atom format in
If the selection requestor is not in this process, \fIformat\fR determines
the representation used to transmit the selection to its
requestor.
.BE
.SH DESCRIPTION
.PP
\fBTk_CreateSelHandler\fR arranges for a particular procedure
(\fIproc\fR) to be called whenever \fIselection\fR is owned by
\fItkwin\fR and the selection contents are requested in the
form given by \fItarget\fR. 
\fITarget\fR should be one of
the entries defined in the left column of Table 2 of the
X Inter-Client Communication Conventions Manual (ICCCM) or
any other form in which an application is willing to present
the selection.  The most common form is STRING.
.PP
\fIProc\fR should have arguments and result that match the
type \fBTk_SelectionProc\fR:
.CS
typedef int \fBTk_SelectionProc\fR(
        ClientData \fIclientData\fR,
        int \fIoffset\fR,
        char *\fIbuffer\fR,
        int \fImaxBytes\fR);
.CE
The \fIclientData\fR parameter to \fIproc\fR is a copy of the
\fIclientData\fR argument given to \fBTk_CreateSelHandler\fR.
Typically, \fIclientData\fR points to a data
structure containing application-specific information that is
needed to retrieve the selection.  \fIOffset\fR specifies an
offset position into the selection, \fIbuffer\fR specifies a
location at which to copy information about the selection, and
\fImaxBytes\fR specifies the amount of space available at
\fIbuffer\fR.  \fIProc\fR should place a NULL-terminated string
at \fIbuffer\fR containing \fImaxBytes\fR or fewer characters
(not including the terminating NULL), and it should return a
count of the number of non-NULL characters stored at
\fIbuffer\fR.  If the selection no longer exists (e.g. it once
existed but the user deleted the range of characters containing
it), then \fIproc\fR should return \-1.
.PP
When transferring large selections, Tk will break them up into
smaller pieces (typically a few thousand bytes each) for more
efficient transmission.  It will do this by calling \fIproc\fR
one or more times, using successively higher values of \fIoffset\fR
to retrieve successive portions of the selection.  If \fIproc\fR
returns a count less than \fImaxBytes\fR it means that the entire
remainder of the selection has been returned.  If \fIproc\fR's return
value is \fImaxBytes\fR it means there may be additional information
in the selection, so Tk must make another call to \fIproc\fR to
retrieve the next portion.
.PP
\fIProc\fR always returns selection information in the form of a
character string.  However, the ICCCM allows for information to
be transmitted from the selection owner to the selection requestor
in any of several formats, such as a string, an array of atoms, an
array of integers, etc.  The \fIformat\fR argument to
\fBTk_CreateSelHandler\fR indicates what format should be used to
transmit the selection to its requestor (see the middle column of
Table 2 of the ICCCM for examples).  If \fIformat\fR is not
STRING, then Tk will take the value returned by \fIproc\fR and divided
it into fields separated by white space.  If \fIformat\fR is ATOM,
then Tk will return the selection as an array of atoms, with each
field in \fIproc\fR's result treated as the name of one atom.  For
any other value of \fIformat\fR, Tk will return the selection as an
array of 32-bit values where each field of \fIproc\fR's result is
treated as a number and translated to a 32-bit value.  In any event,
the \fIformat\fR atom is returned to the selection requestor along
with the contents of the selection.
.PP
If \fBTk_CreateSelHandler\fR is called when there already exists a
handler for \fIselection\fR and \fItarget\fR on \fItkwin\fR, then the
existing handler is replaced with a new one.
.PP
\fBTk_DeleteSelHandler\fR removes the handler given by \fItkwin\fR,
\fIselection\fR, and \fItarget\fR, if such a handler exists.
If there is no such handler then it has no effect.
.SH KEYWORDS
format, handler, selection, target
